To do this, you need to full circuit length (2x the distance from the battery to the fuse block, "as the wire runs"). You'll also need the max draw of the fuse block. You can find charts on the internet that give you the resistance per foot for various wire sizes. From there, a bit of E=IR (Voltage = Current * Resistance) arithmetic tells you the voltage drop to expect. You then divide this into 12V (I use 12.8v) to get the % drop.

The results typically result in wire a size or two larger than typical charts tell you.

If your fuse block supports it, you should also consider running both a + and - cable from the battery to the fuse block. (Only the + needs to be fused.) This gives you a "clean", "known-good" ground, and ensures that your sizing calculations are correct (body grounds can have surprisingly high resistance).